Re: [PATCH 3/2] Avoid unnecessary strlen() calls

From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Mar 20, 2007, 04:11 UTC
Message-ID: <Pine.LNX.4.64.0703192052380.6730@woody.linux-foundation.org>
In-Reply-To: <20070320034020.GB29145@spearce.org>

On Mon, 19 Mar 2007, Shawn O. Pearce wrote:

>
> DrNick suggested on #git to try flipping the isspace test around.
> This is a smaller change and generated the same ~3.60 seconds run
> as Dscho's patch.  I like DrNick's version better.  ;-)

For me, the result seems to be in the noise.

It may be due to running on Core 2. It's not very sensitive to micro-optimizations like this. It definitely makes sense to test the *stable* test first, since that will help branch prediction (the "isspace()" test is *not* very predictable), so I don't disagree with the patch, but I suspect it depends a lot on the microarchitecture just how much it matters.

Do you perhaps have a P4? It has a very bad branch mispredict penalty, so putting the predictable branch first could explain the big difference you see..

Dscho's bigger patch probably helps more on an in-order architecture, and should be equally good on a P4 (or Opteron). On Core 2, neither of the patches seem to make a huge difference.

			Linus